1. Field of the Invention
The present invention relates to a photo detector for irradiating a sample with a light source light and detecting the scattered light with a row of space light receiving elements, and belongs to the technical field of equipment for measuring a particle size distribution in a sample.
2. Description of Related Art
A diffracted light (scattered light) detector in which a plurality of light receiving elements such as photo diodes formed as circular arcs are arranged in a fan shape about a concentric circle is disclosed, for example, in Japanese Patent Publication No. Hei 2-145940.
The cross section of the light receiving element is shown, for example, in FIG. 4, where numeral 1 denotes a P layer in which the positive charge collects, numeral 2 the N layer in which the negative charge collects, numeral 3 the N+ layer, numeral 4 a depletion layer, numeral 5 a positive electrode, numeral 6 a transmittable insulation layer, and numeral 7 a negative electrode. The intensity of light received in the above-mentioned P layer 1 is taken out as a meassurement in the form of photoelectric current from the positive electrode 5.
The above-mentioned P layer 1 constitutes each light receiving element b, c, d, . . . which is, in general, formed in oblong circular arcs in which each light receiving element is arranged on a concentric circle to form a fan shape (see FIG. 5), and photoelectric current corresponding to the light intensity in accord with each scattering angle is detected. The light receiving element a at the center portion detects the transmitted light.
On the circumferential portion of the P layer 1, a depletion layer 4 is formed but the shape of depletion layer 4 is extremely difficult to establish from the viewpoint of manufacturing, and the interface 1a between the P layer 1 and depletion layer 4 is usually formed inclined so as to spread in front. In the interface 1a, it is difficult to establish a precise shape, and about several .mu.m manufacturing errors tend to occur during production.
When the shape accuracy of the interface 1a of P layer 1 is low, the accuracy of the arrangement condition between light receiving elements, for example, the accuracy of the isolation gap and alignment or the accuracy of effective light receiving area of each light receiving element degrades, causing a problem in that a high measurement accuracy cannot obtained.
Because it becomes difficult to arrange high shape accuracy light receiving elements at the position near the center portion, the measurement range of an area with small scattering angles is restricted, causing a problem in that the measurement range of a large-particle size area is limited, and it has been extremely difficult to expand the measuring area while achieving downsizing or higher accuracy.